As many as 40,000 barrels (1.7 million gallons) of oil a day may have been gushing out from a blown-out Gulf of Mexico well, doubling many estimates.
New estimates for the oil spill prior to 3 June follow an assessment by a team of scientists put together by the US government and co-ordinated by the US Geological Survey.
White House energy adviser Carol Browner said the US was "prepared for the worst scenario" that the leak might not be stopped before August.
At least 20 million gallons have now spilled into the Gulf of Mexico, affecting more than 70 miles (110km) of Louisiana's coastline.
What makes this region ecologically special is the unusual patterns of land and sea conjured by the variegated exit of the Mississippi into the Gulf.
This area includes 25% of US wetlands - rich with life, where human occupancy is low, and birds and other animals thrive.
"For birds, the timing could not be worse; they are breeding, nesting and especially vulnerable," warns Melanie Driscoll, a Louisiana-based bird conservation director with the National Audubon Society.
Some Americans have protested against the perceived inaction of officials. Commercial fisherwoman Diane Wilson of Seadrift, Texas, demonstrated at a Senate Energy and Natural Resources Committee hearing in Washington.
The Obama administration has come under increasing pressure to find a way to mitigate the environmental and economic impact on the Gulf area. The president vowed to end the "cosy relationship" between oil companies and US regulators.
He condemned "the ridiculous spectacle" of them "falling over each other to point the finger of blame".
BP said the device placed on the well collected 15,800 barrels of oil on Wednesday - slightly up on the 15,010 barrels collected in the previous 24-hour period.
Gas and oil continue to leak from the containment cap at the Deepwater Horizon site.
